A laminated glass is a safety glass because few glass fragments are scattered even if it is broken by impact from the outside. Therefore, laminated glasses have been used widely for windowpanes of motor vehicles such as automobiles and railroads, aircrafts, buildings, and the like. One example which can be mentioned is a laminated glass which includes an interlayer film for a laminated glass interposed between and integrated with one pair of glass plates. The interlayer film for a laminated glass consists of a polyvinyl butyral resin and a plasticizer.
Laminated glasses that effectively insulate driving noise of an engine, wind noise, and other external noises have been demanded in recent years. As a laminated glass excellent in sound insulation, Patent Document 1, for example, discloses a laminated glass with an interlayer film for a laminated glass. Here, the laminated glass has two or more layers of plasticized polyvinyl acetal resin films laminated together.
Examples of a method for producing an interlayer film for a laminated glass excellent in sound insulation include a method in which a polyvinyl butyral resin with low hydroxyl group content is used, a method which includes adding an increased amount of a plasticizer to a polyvinyl butyral resin, and a method which includes adding a low-molecular-weight plasticizer.
However, use of the method in which a polyvinyl butyral resin with low hydroxyl group content is used causes such problems as deterioration of moisture resistance of a laminated glass. In addition, the interlayer film for a laminated glass that contains a polyvinyl butyral resin with low hydroxyl group content has only insufficient strength. Thus, penetration resistance of the laminated glass is problematically deteriorated.
In the method including adding an increased amount of a plasticizer to a polyvinyl butyral resin, or the method including adding a low-molecular-weight plasticizer, the plasticizer bleeds out onto the surface from the inner side of the interlayer film. This problematically causes difficulty in controlling the adhesion between an interlayer film for a laminated glass and glass plates. Also problematically, the increased amount of the plasticizer significantly reduces penetration resistance of a laminated glass.